resource & predation controls on ecosystem production
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Resource & Predation Controls on Ecosystem Production
Bottom-up Controls – refer to control of abundance or productivity of a species or functional group by supply of resources.
What controls production in an ecosystem?
The “bottom up” view of Ecosystem
production
Nutrients
Global Ocean distribution of Chlorophylland Benthic Faunal Biomass mg/m3
“Trickle Up” effectIncreases in primary production propagate
through the food web
What controls production in an ecosystem?
The “bottom up” view of Ecosystem
production
The “top down” view of Ecosystem production
Nutrients
Basic Premise:“Any population which is not resource limited must, of course, be limited to a level below that set by its resources.”
Therefore the “usual condition is for populations of herbivores not to be limited by food supply….” and producers are limited by resources, not herbivores
Top-Down Control & Inferences about Trophic Cascades
Hairston, Slobodikin and Smith 1960
“Predators and parasites in controlling herbivores … must be food limited.”
World is Green
But, plants may become depleted whenever herbivores become numerous enough (insect outbreaks, e.g. adelgid)
The remaining general method of herbivore control is predation”
Nutrients
“Green World Hypothesis”
Limited by prey availability
Limited by predation and parasitism
Limited by nutrient availability, not herbivory
Nutrients
“Green World Hypothesis”
Although the authors did not explicitly call this series of interactions a “trophic cascade” , that was the idea that was implicit in the Green World Hypothesis.
Systems with and without predators
Brooks and Dodson (1965) examined 9 lakes:
•Four lacked Alewifes•Four had natural Alewife populations•One had introduced Alewifes • sampled before and after the introduction
Large zooplankton out compete small zooplankton. Populations of small zooplankton increase in
abundance when large zooplankton are depleted.
Size-efficiency Hypothesis. Larger zooplankton more effectively graze phytoplankton than smaller zooplankton. Therefore if zooplanktivorous fish are absent, large herbivorous zooplankton will increase and phytoplankton abundance will be reduced.
Zooplankton Size Distribution in Lakes with:
SMALLBosmina Tropocyclops Cyclops
DOMINANT ZOOPLANKTERS
CeriodaphniaAsplanchna
Alewife
LARGE
DOMINANT ZOOPLANKTERS
Dioplomus CyclopsDaphnia Mesocyclops
Lepiodore5 mm
Epischura
No Alewife
A “Trophic Cascade”
+ Bass
- Bass
Control
Food web manipulations inPeter, Paul and Tuesday Lakes
If piscivores added to lake, they will eat and deplete zooplanktivorous fish.
Populations of larger herbivorous zooplankton will grow.
Phytoplankton populations will be reduced.
Food web manipulation experiments of in 3 lakes (from Carpenter et al., 1987. Ecology 68:1863-1876).
Control
+Bass
-Bass
Lakes with bass (piscivore added), zooplanktivorous fish depleted, large zooplankton increase, chl a is lower
Lakes with piscivore removed have higher chl a
ZOOPLANKTON PHYTOPLANKTON
Sea Otters and urchin grazers
And Killer Whales
TROPHIC CASCADE IN KELP ECOSYSTEMS
Effect of Killer whale predation on Sea Otters
From Estes et al. 1998 Science 282:473
No. p
er 0
.25
m2
10 8 6 4 2 0 1972 1985 1989 1993 1997
Year
Total Kelp Density
400300200100 0
gms
0.25
m
-2
Sea Urchin Biomass
Amchitka I.N. Adak I.Kagalaska I.L. Kiska I.#
Otte
rs (%
max
co
unt)
100
80
60
40
20
0
Sea Otter Abundance
Grazing Intensity
605040302010 0
% L
oss
24
hr -1
HP
-
C1
-+
C2
-+-
Trophic Cascades and Feedbacks
C3
-+-+
Links1 (odd)
2 (even)
3 (odd)
4 (even)For simple food chains:An odd number of trophic links results in control of primary producers by grazing (top-down)An even number of trophic links results in control of primary producers by resources (bottom-up)
And nutrients or other factors limiting producers can still increase producer biomass and have effects that
propagate up the food webBoth Top-down and Bottom-Up controls influence
abundance and productivity of populations
CX But organisms feeding at multiple trophic levels can complicate picture
Nutrients
P
Are Trophic Cascades All Wet?
Reasons Why Trophic Cascades Might be Stronger in Aquatic Ecosystems than in
Terrestrial Ecosystems
Herbivores are bigger relative to plants in aquatic ecosystems (e.g. zooplankton vs. phytoplankton compared with insects vs. trees)
Aquatic primary producers are more nutritious and have element composition more like their herbivores
Terrestrial plants have lower P:B ratios than aquatic plants (is this a cause or a consequence??)
Aquatic herbivores consume about 3X more autotrophic production than terrestrial herbivores (but why??)
Comparison of Herbivory in Aquatic and Terrestrial Ecosystems
(Cyr and Pace, 1993 Nature 361:148)
Frequency distributions showing the proportion of NPP removed by herbivores in ecosystems with different primary producers
Median, 18%
Median, 30%
Median, 79%
Regardless of amount of NPP, rates of Herbivory are on average about 3X higher in Aquatic than Terrestrial Ecosystems (note:
log scale)
Comparative Strength of Trophic Cascades Across Ecosystems based on Manipulations of Predators
in Six Types of Ecosystems (102 studies)Plot loge (Abundance+predators/Abundance-predators)
(modified from Shurin et al. 2002, Ecology Letters 5:785)
Ratio
B+p
reda
tors/B
-pre
dato
rs fo
r Pl
ants
0.7% 1.8% 5.0% 13.5% 36.8% 100%
20.0
7.4
2.7
o Herbivore response greatest in lentic & marine systems (e.g. density with predators ~6% to13% of that without predators) and least in terrestrial (density + predators ~71% of density - predators)
o Plant response greatest in marine benthos (biomass 4.7X > in systems with predators) vs. terrestrial (1.1X > with predators)
o Across systems, as Herbivores ⇩ plant biomass response ⇧
Reasons why we might not ‘see’ top down cascades in land
ecosystemsMany terrestrial plants have complex structural tissue that is harder to digest and have evolved ‘anti-herbivore’ compounds
Perhaps grazing in terrestrial ecosystems is inhibited by Antiherbivore Compounds/Lignin Derivitives
Effect of Tannins on palatability of grass to Geese
Re l
ativ
e A
mo u
n t E
aten
From Buchsbaum et al
Reasons why we might not ‘see’ top down cascades in land
ecosystems
• Many terrestrial apex predators have been hunted to near or local extinction
WolvesRe-introduced
Ripple et al. 2006. For. Ecol & Mgt. 230:96
Per
cent
Bro
wsin
g
Willo
w He
ight
(cm
)100
50
0400
200
098 99 00 01 02 03 04 05
WOLVES AND WILLOWS
2-3 m3-4 m
<2 m
Reasons why we might not ‘see’ top down cascades in terrestrial
ecosystems• Plants have complex tissues and anti-herbivore
compounds
• Terrestrial may have more complex and more detritus based food webs, less direct grazing.
• Many terrestrial apex predators have been hunted to near or local extinction
• Longevity of the plant community (decades to centuries for mature plants) makes it hard to measure the results
• Terrestrial ecosystems are less experimentally tractable than their aquatic counterparts, in part because of extreme longevity of the plant community
• Many of the more charismatic species now enjoy stringent legal protection, which hampers manipulation;
"The ecology of fear" • Defensive strategies to avoid
predation• Reduced energy intake• Alternate food sources • Increased susceptibility to other
predators• Lower mating success • Emigration
No Trophic Cascade : Creek Infauna Abundance was lower
Fleeger et al. Accepted JEMBE
Tota
l Ann
elid
s (#
/ m
2 x
10 3
)
0
20
40
60
Reference Nutrient-FishFish -FishFish
Expected
Observed
Mummichog Reduction
MummichogInvertsAlgae
Behavior alters species roles in ecosystem
C1HP
-
Nutrients
P
MummichogInvertsAlgae(Shrimp)
6
8
10
12
Shr
imp
d 15
N
Fish -Fish Fish -Fish
Reference Nutrient enrichment
Removal of mummichogs allows shrimp to forage in more open areas and become more
carnivorous.
Behavior alters species roles in ecosystem
David JohnsonKari GalvanLinda Deegan
SUMMARY• Top down controls can be important
in ecosystem dynamics• Can affect more than just the level
below (cascade), including effects on adjoining ecosystems
• Interact with nutrient level• Indirect trait mediated effects may
be as important as direct consumption effects
• Stronger in Aquatic or Terrestrial? No consensus just yet.
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